WHY AREN'T WE DOING THIS? THE VALUE OF CIRCULAR POLARIZATION IN PETROGRAPHY AND PHOTOMICROSCOPY

Conventional microscopic examination of rocks in thin section is conducted with plane-polarized and cross-polarized transmitted light. Another mode of polarization, circular polarization, is available which offers certain advantages for mineral identification and characterization, and photomicroscopy.

In circular polarization a plane-polarized ray is decomposed into two plane-polarized rays whose vibration directions (electric vectors) are at 90° to one another, with one ray retarded by a quarter wavelength behind the other. When added vectorially the resultant electric vector spirals as the ray propagates. Instrumentally, circular polarization is achieved by placing a quarter-wave plate above the polarizer but below the stage, and another quarter-wave plate above the stage and below the analyzer.

The value of circular polarization in petrography is that all mineral grains will exhibit their highest order interference colors, given orientation of crystallographic axes relative to the plane of the stage, regardless of stage orientation. That is, there is no change in interference colors when you rotate the stage. This facilitates rapid recognition of two kinds of grains that are useful for mineral identification and characterization: (1) grains of a given mineral exhibiting the highest order interference color, useful for determining birefringence; and (2) grains of a given mineral exhibiting the lowest order interference color, useful for obtaining a centered optic axis figure. Circular polarization also allows us to more readily distinguish isotropic minerals, glass, and voids from mineral grains that just happen to be at extinction. Circular polarization has value for photomicroscopy because all anisotropic mineral grains, with the exception of those with an optic axis perpendicular to the stage, exhibit interference colors ; none are black simply due to stage orientation, and, therefore, more information is conveyed. Circular polarization can be readily brought into the mineralogy/petrology classroom. Zeiss and Leica currently offer polarizing microscopes that are easily adapted to obtain circular polarization.